1. Field of the Invention
This invention generally relates to methods of reclaiming large amounts of calcium sulfite, as is produced by lime-based flue gas desulfurization (FGD) systems. More particularly, this invention is directed to a process and equipment capable of being retrofitted to an existing FGD facility at minimal cost and with minimal interruption of the existing scrubbing operation, and which efficiently converts the stock of calcium sulfite produced by the FGD facility to calcium sulfate.
2. Description of the Prior Art
A significant number of facilities exist that produce and/or store large amounts of calcium sulfite. Primary examples are lime-based flue gas desulfurization (FGD) systems that remove substances such as acidic constituents and particulate matter from combustion or flue gases produced by utility and industrial plants. Limebased flue gas desulfurization processes have typically involved the use of limestone (calcium carbonate; CaCO.sub.3) slurries or hydrated lime (calcium hydroxide; Ca(OH).sub.2) slurries formed by action of water on lime (calcium oxide; CaO). Intimate contact between these calcium-based slurries and acidic gases that are present in flue gases, such as sulfur dioxide, hydrogen chloride (HCl) and hydrogen fluoride (HF), result in the absorption of the gases by the slurry and the formation of salts, such as calcium sulfite (CaSO.sub.3.1/2HOH), calcium sulfate (CaSO.sub.4.2HOH), calcium chloride (CaCl.sub.2 ) and calcium fluoride (CaF.sub.2) precipitates, which must be collected for disposal, recycling or sale.
Calcium sulfite is the primary byproduct of lime-based FGD operations, but has no commercial value and is difficult to dispose of. In contrast, calcium sulfate (gypsum) is a saleable byproduct, is significantly easier to handle, and can be more readily stored long-term. In more modem FGD facilities, scrubber units are adapted for in situ forced oxidation of the slurry (typically within the gas-liquid contactor) to ensure that all of the sulfites are reacted to form sulfates, thereby maximizing the production of gypsum. However, many existing facilities are not equipped for in situ forced oxidation, and as a result accumulate large amounts of calcium sulfite slurry that is typically stored for long periods in holding ponds prior to disposal. During storage, calcium sulfite can leach into the surrounding soil, posing a hazard to plant life and ground water. As a result, processes have been proposed for reclaiming and converting calcium sulfite in holding ponds. One such method utilizes a liquid source of acid that is introduced into a storage tank into which the slurry is pumped. An amount of acid is added to sufficiently lower the pH of the mixture to enable the sulfite precipitate to become dissolved in the mixture, which is then aerated with compressed air to oxidize the calcium sulfite and produce calcium sulfate. This type of system is expensive to operate, primarily due to the cost of the acid when compared to the value of the final sulfate product.
In view of the above, it would be desirable if a method were available that was practical for reclaiming and converting calcium sulfite slurry accumulated from lime-based flue gas scrubbing operations that do not have the capability for in situ oxidation. To be practical, such a method must be capable of being adapted for use with existing FGD facilities without requiring major renovations to the existing FGD scrubber units, capable of being installed and commenced with minimal interruption of the scrubbing process, and operate without the necessity for adding an additional acidification agent.